The goals of this research are to investigate the mechanism of tyrosine phosphorylation in Gsk-3 family proteins using yeast as a model system. Gsk-3beta homologues have been linked to developmentally important pathways in cell signaling such as Wnt and Hedgehog in vertebrates and Drosophila as well as degradation, meiosis, sporulation and osmoregulation in S. cerevisiae. Gsk-3beta family members contain a highly conserved tyrosine in the activation loop and phosphorylation of this tyrosine appears to activate Gsk-3beta ser/thr kinase activity. Currently few details are known about the general mechanism of Gsk-3betaa tyrosine phosphorylation. Specifically, I propose to investigate the phosphorylation of tyrosine 216 in Ygk3. I will determine the phosphorylation state of Ygk3 under various cell-growth conditions as well as determine if tyrosine 216 phosphorylation is essential of Ygk3 activity. I will also find the yeast kinases that target Ygk3 for tyrosine phosphorylation on tyrosine 216. This information will be useful for constructing a model of Gsk-3beta activity in yeast as well as more highly evolved organisms.